An electrochemical measuring cell of this kind is disclosed in U.S. Pat. No. 3,649,505 and includes a pH-electrode as a measuring electrode which is used to measure hydrogen ions. This potentiometric measurement of an ammonia concentration requires a long time duration for a completed measuring reaction. The long time duration is needed for the adjustment of an equilibrium. In this time duration, the NH.sub.3 to be detected and the water content of the electrolyte conjointly form NH.sub.4 OH which, in turn, dissociates into NH.sup.+.sub.4 ions and OH.sup.- ions. The slow step determining the speed for this reaction is the adjustment of the equilibrium with the gas space or the adjustment of the equilibrium at the glass membrane.
The glass electrode required for the pH-measurement changes in the characteristic of the glass membrane in the course of its use so that drift phenomena occur. A stable reference potential is necessary for carrying out the pH-measurement and a displacement of this reference potential in the course of use likewise leads to drift phenomena. The known measuring cell responds to all gases influencing the pH-value of the electrolyte so that its selectivity for measurements in corresponding gas mixtures is not adequate.